This invention relates to controllable pitch aircraft propellers and more particularly to such propellers which are adapted to be operated with free turbine engines having a beta pitch control mechanism, such as the Pratt & Whitney PT 6A engine series, for example. More particularly, the invention relates to reversible pitch turbo-propeller which is capable of automatically providing a blade shift to a lower pitch angle, upon command of the pilot during ground operations, for the purpose of maintaining the rpm of the turbine and the thrust of the propeller within acceptable limits and for assuring that the propeller does not operate in the reactionless mode.
The propeller control system to which the present invention is applied is basically the system as shown in U.S. Pat. No. 3,249,159 issued May 3, 1966 to Biermann and U.S. Pat. No. 3,446,289 issued May 27, 1969 to Morris, Jr., in which the blade position is transmitted to a valve (a beta valve) through a mechanical feedback arrangement carried on the propeller. While the control system illustrated herein is applied to an external feedback propeller, of the general type shown in Biermann U.S. Pat. No. 3,249,159 as noted above, it should be understood that the invention is not limited to this particular concept and may be applied, for example, to a propeller with internal feedback to a beta rod such as shown in Biermann U.S. Pat. No. 2,986,222 issued May 30, 1961 or Biermann U.S. Pat. No. 3,380,535, issued Apr. 30, 1968.
During the approach conditions, if the pilot selects an approach or flight idle position, such as during flare out and touchdown, the propeller blades are normally maintained by the system at a minimum positive blade angle. This angle has been selected to provide relatively low thrust or drag, taking into account the approach speed of the aircraft, and the minimum gas generator speed required to sustain the operation of the gas turbine. However, once the aircraft has landed and its landing speed has been largely dissipated, the propeller blade angle is often high enough to cause the propeller rotation speed to be lowered into the speed range of a reactionless mode. This reduction in propeller rpm during taxi operation is due to the nature of the free turbine engine in that the propeller and free power turbine speed is a function of blade angle and gas generator speed. However, it is not practical to permit the propeller speed to drop below a minimum level at which a reactionless mode may be encountered.
The term "reactionless mode " describes a resonance vibration condition which can affect propellers having four or more blades, during ground operation. Generally, the reactionless mode condition is more likely to occur during ground operations in crosswinds or in quartering tailwinds, and during times that the propeller speed has dropped into a critical range. Each propeller design having four or more blades is characterized by known or predictable conditions at which the reactionless mode may be encountered. Since the reactionless mode results in high blade and hub stresses, without producing any net load to the engine flange, it is a condition to be avoided whenever possible. One way of avoiding this condition is to provide a system which is capable of decreasing the blade angle, at or following landing or rollout, either automatically or at the command of the pilot, to a point where the blade will continue to be driven by the turbine at an rpm safely above that range in which the reactionless mode may occur. For example, a propeller may be physically designed to encounter reactionless mode conditions in the range of 700 to 1,000 rpm, and the control system may be designed to provide a power setting and/or blade setting which assures that the propeller speed does not fall below 1,050 rpm during ground operations. However, simply adding fuel to the turbine to maintain such propeller speeds, without further descreasing the blade angle from the minimum approach configuration, commonly results in substantial excess thrust, thereby causing problems for the pilot in taxiing and the like.
In an initial attempt to deal with this condition, aircraft engine manufacturers have added a selectable position to the power levers, namely that of ground idle. The control quadrant is provided with a stop or gate arrangement which normally intercepts the power levers at the flight idle minimum position. However, the pilot has the means of moving the power levers in such a way as to bypass this stop or gate into a ground idle range, usually by grasping and lifting the levers over the gate. During this movement, a mechanical cam is brought into operation which physically moves the beta valve in such a direction as to cause the propellers to decrease pitch by a small amount, such as about 4.degree.. At the same time, a connection is made by the cam to the fuel control arm of the turbine fuel control unit which may cause the fuel control unit on the engine to deliver the specified amount of turbine fuel to the engine to assure that it maintains its minimum operating speed during ground operations.
Airframe or propeller manufacturers have found that the propeller idle cam arrangement does not by itself move the beta valve sufficiently to cause a sufficient decrease in blade angle. This is due to the fact that the amount of movement available between flight idle and ground idle positions, the amount of cam surface available, and the forces required to move the cam physically restrict the same to about a maximum of 8.degree. movement of pitch angle. Accordingly, both engine and airframe manufacturers have devised additional arrangements for imparting further blade movement to lower pitch settings following landing.
One arrangement which has come into relatively wide use is that employed by the Beech Aircraft Company in its King Air models, consisting of a squat-switch operated electric solenoid. The switch may be directly operated oy the pilot or it may be indirectly operated by sensing a power lever position in the ground idle range or it may be automatically operated by means of a connection to the landing gear and be operated when the landing gear has contacted the ground and sufficient weight has been transferred to the wheels. In any case, a squat-switch actuates an electric solenoid coupled to the beta valve and effects a predetermined additional shift to this valve, independent of the movement of the power levers into the ground idle range, causing the propeller blades to move to a lower and acceptable pitch angle for ground operation, as compared to the higher approach angle.
An additional system has been designed by Pratt & Whitney which is pneumatically operated oy bleed air from the compressor, to operate a pneumatic piston which, like the Beech system, is mechanically connected to reopen and move the beta valve through a finite distance. The application of bleed air to the piston is again either controlled or operated by a squat-switch on the landing gear or operated by a pilot command switch.
In either the Beech or the Pratt & Whitney systems, the means by which the blade angle is reduced consists of an external actuator which is connected to move the beta valve thus reopen the same to cause additional movement of the propeller pitch changing mechanism to a lower pitch angle until the movement is transferred back by the pitch change mechanical feedback mechanism to the beta valve, at which point the valve is reclosed. In both systems, the blade angle is reduced to raise propeller speed out of the range of reactionless mode while leaving the gas generator at its minimum sustaining power level or speed, as outlined above.
Since the existing systems are operated generally external to the propeller, and require a separate source of power for operation, such as bleed air and/or electric power, there exists a need for a completely self-contained beta shift mechanism operated within the propeller pitch change mechanism itself.